Field of the Invention
The present invention relates to organic compounds and photoelectric conversion elements.
Description of the Related Art
In recent years, the power required for driving electric circuits has become extremely low. Preparing for the coming IoT society, various electronic parts, such as sensors, have been able to be driven with very low electric power (order of μW). As for utilization of a sensor, applications for an energy harvesting element has been expected as a self-sufficient energy supply capable of generate and consume power in-situ. Among the energy harvesting elements, photoelectric conversion elements have been attracted attentions as an element capable of generating power anywhere as long as there is light. As an energy harvesting element, particularly demanded is a photoelectric conversion element capable of efficiently generating electric power with weak light. Typical examples of weak light include LED light, and light of a fluorescent lamp. These are typically used indoor, and are called indoor lighting. The illuminance of these types of light is from about 20 lux through about 1000 lux, and these are very weak light compared to direct sunlight (about 100,000 lux). As an energy harvesting element, there is a need for an element capable of efficiently generating energy with indoor lighting, such as a fluorescent lamp, and an LED lamp.
As the photoelectric conversion element, the most widely used is a silicon-based battery cell. Various silicon-based battery cells having high conversion efficiency under sunlight have been reported (for example, Panasonic Electric Works Technical Report, 56 (2008) 87). However, it has been generally known that the silicon-based solar cell has low conversion efficiency under weak light, through the silicon-based solar cell has excellent conversion efficiency with sun light (for example, Nature, 353 (1991) 737). Meanwhile, it is reported that a dye-sensitized solar cell presented by Graetzel et al. of Swiss Federal Institute of Technology in Lausanne has photoelectric conversion properties better than silicon solar cells, under weak light (see, for example, J. Am. Chem. Soc., 115 (1993) 6382). Moreover, it is also known that a bulk heterojunction organic thin solar battery, in which a p-type organic semiconductor developed by Heeger et al., and a n-type organic semiconductor, such as fullerene, are mixed, has relatively high power generating ability with weak light (Adv. Mater. 2013, 25, 2397-2402).
It is known associated with properties of a photoelectric conversion element that open-circuit voltage is typically largely reduced, as the light intensity is reduced. This reduction in the open-circuit voltage is a significant factor for degrading the properties of a solar cell under weak light. The tendency mentioned above is also applied for conventional organic thin-film solar cells. Therefore, there is a need for improving the low open-circuit voltage under weak light.
Moreover, it is known that a short-circuit current density, which is one of properties of a photoelectric conversion element, is proportional to a light intensity, if the light source for use is the same. This tendency as mentioned is also applied for a so-called organic thin-film solar cell. Conventional organic thin-film solar cells have been developed to use sun light as a light source. Among them, development of p-type organic semiconductors has been particularly actively conducted.
Meanwhile, a photoelectric conversion element, a light source of which is indoor lighting, needs to exhibit a high electric current value with a fluorescent lamp, or an LED lamp, because the light used is a fluorescent lamp or an LED lamp, not sunlight. Unlike sunlight, light emitted from a fluorescent lamp or LED lamp has a spectrum only in visible light region. A p-type organic semiconductor used in a conventional photoelectric conversion element designed to use with sunlight does not match with the spectrum of the fluorescent lamp or LED lamp very well, and thus use of such the p-type organic semiconductor in the photoelectric conversion element leads to a low electric current value with light of a fluorescent lamp or LED lamp. Accordingly, it is desired to develop a material suitable for a spectrum of light of a fluorescent lamp or LED lamp. Specifically, there is a need for a material having an absorption spectrum present at the shorter wavelength side to an absorption spectrum of a p-type organic semiconductor designed to use with sunlight.
The literature “Chem. Mater. 2013, 25, 2274-2281” discloses a material whose absorption wavelengths are relatively short wavelengths, and discloses an organic material exhibits a relatively high electric current value under simulated sunlight. However, this literature does not teach properties of the material with low illuminance.
Moreover, a p-type organic semiconductor used in a bulk heterojunction organic thin-film solar cell is formed into a film through a solution coating process. Therefore, the p-type organic semiconductor is desired to have high dissolvability. Especially in the case where it is desired to make a film thickness large, and sufficiently perform light absorption, the higher dissolvability of the p-type organic semiconductor is required. In order to adjust absorption wavelengths of a low-molecular p-type organic semiconductor, moreover, a method for introducing an acceptor segment is often used. In this case, the dissolvability of the p-type organic semiconductor tends to be lowered, and thus there is a problem where the p-type organic semiconductor is not dissolved in a solvent used for film formation. For the purpose of solving the aforementioned problem, a long-chain alkyl group is introduced into a skeleton of the p-type semiconductor. In the aforementioned literature, an acceptor segment is introduced, and a long-chain alkyl group is introduced to secure dissolvability.
However, the aforementioned conventional organic material does not have sufficient dissolvability, and hence cannot be suited for formation of a thick film.